Two-cycle combustion engine of air scavenging type

ABSTRACT

An engine including first scavenging passages for fluidly connecting between a combustion chamber and a crank chamber through bearings for the crankshaft, second scavenging passages fluidly connecting directly between the combustion chamber and the crank chamber, a suction chamber formed in a side face of a piston. An air-fuel mixture is sucked into the suction chamber and an air is introduced into the crank chamber. During the intake stroke, the air-fuel mixture from the air-fuel mixture passage is introduced into the first scavenging passages through the suction chambers, and the air is introduced into the crank chamber. During the scavenging stroke, before the air-fuel mixture in the first scavenging passages is introduced into the combustion chamber, introduction of the air in the crank chamber into the combustion chamber through the second scavenging passages starts.

FIELD OF THE INVENTION

The present invention relates mainly to a two-cycle internal combustionengine of an air scavenging type that is used as a drive source for acompact rotary machine such as, for example, a brush cutter.

BACKGROUND ART

The conventional combustion engine of this kind is known to be sodesigned that, prior to scavenging of a combustion chamber with anair-fuel mixture, the combustion chamber is initially scavenged with anair to suppress the blow-off of the air-fuel mixture through an exhaustport. (See, for example, the Japanese Laid-open Patent Publication Nos.2001-173447 and 58-5424.)

It has been found that while in the two-cycle combustion engine of thisair scavenging type, bearings disposed in the engine cylinder block forthe support of the crankshaft are lubricated with the air-fuel mixtureintroduced into the crank chamber, an attempt to make the combustionengine of this kind compact tends to reduce the size of a gap throughwhich the air-fuel mixture within the crank chamber to such an extent asto make it difficult to lubricate the bearings. For this reason,formation of oil supply passages to lubricate the bearings effectivelyrenders the engine structure complicated.

DISCLOSURE OF THE INVENTION

In view of the foregoing, the present invention has for its object toprovide a two-cycle combustion engine in which a fluid circuit throughwhich an air-fuel mixture can flow by way of bearings so that thebearings can be sufficiently lubricated with a simplified structure.

In order to accomplish the foregoing object, the two-cycle combustionengine according to a first aspect of the present invention includes afirst scavenging passage for communicating between a combustion chamberand a crank chamber through a bearing for a crankshaft, a secondscavenging passage for communicating directly between the combustionchamber and the crank chamber, a suction chamber formed in a side faceof a piston, an air-fuel mixture passage for introducing an air-fuelmixture M into the suction chamber, and an air passage for introducingan air into the crank chamber, and is so designed that during an intakestroke of the engine, the air-fuel mixture from the air-fuel mixturepassage can be introduced into the first scavenging passage through thesuction chamber and the air from the air passage is introduced into thecrank chamber, and that during a scavenging stroke of the engine,introduction of the air within the crank chamber into the combustionchamber through the second scavenging passage can take place before theair-fuel mixture within the first scavenging passage is introduced intothe combustion chamber.

With this two-cycle combustion engine, when the air-fuel mixture isintroduced from the first scavenging passage into the crank chamberduring the intake stroke, or when the air-fuel mixture within the crankchamber is introduced from the first scavenging passage into thecombustion chamber during the scavenging stroke, such air-fuel mixtureflows through the bearing for the crankshaft. In other words, the pathof flow of the air-fuel mixture through the bearing is established.Accordingly, the bearing for the crankshaft can be satisfactorilylubricated with a simple structure, by a fuel contained in the air-fuelmixture. Also, during the scavenging stroke, prior to the air-fuelmixture within the first scavenging passage being introduced into thecombustion chamber, the air introduced into the crank chamber during theintake stroke can be introduced into the combustion chamber through thesecond scavenging passage. In other words, initial scavenging takesplace with the air first introduced into the combustion chamber,followed by the scavenging with the air-fuel mixture and, therefore, theblow-off of the air-fuel mixture can be satisfactorily suppressed.

The two-cycle combustion engine according to a second aspect of thepresent invention includes a first scavenging passage for communicatingdirectly between a combustion chamber and a crank chamber, a secondscavenging passage for communicating between the combustion chamber andthe crank chamber through a bearing for a crankshaft, a suction chamberformed in a side face of a piston, an air passage for introducing an airinto the suction chamber, and an air-fuel mixture passage forintroducing an air-fuel mixture into the crank chamber, and is sodesigned that during an intake stroke of the engine, the air from theair passage is introduced into the second scavenging passage through thesuction chamber and the air-fuel mixture from the air-fuel mixturepassage is introduced into the crank chamber, and that during ascavenging stroke of the engine, introduction of the air within thesecond scavenging passage into the combustion chamber takes place beforethe air-fuel mixture within the crank chamber is introduced into thecombustion chamber through the first scavenging passage.

This two-cycle combustion engine may be considered having a path of flowof the air-fuel mixture and the air, which is substantially reverse tothat in the two-cycle combustion engine according to the first aspect ofthe present invention. More specifically, it is featured in that duringthe intake stroke the air-fuel mixture is introduced directly from theair-fuel mixture passage into the crank chamber and the air isintroduced into the second scavenging passage from the air passage. Withthis two-cycle combustion engine, since during the scavenging strokethis air-fuel mixture within the crank chamber flows through the bearingfor the crankshaft when a portion of the air-fuel mixture within thecrank chamber enters the second scavenging passage, the bearing for thecrankshaft can be sufficiently lubricated with a simple structure. Also,since during the scavenging stroke the air introduced into the secondscavenging passage during the intake stroke is introduced into thecombustion chamber prior to the air-fuel mixture being introduced fromthe first scavenging passage into the combustion chamber, the blow-offof the air-fuel mixture can be satisfactorily suppressed by the airfirst introduced into the combustion chamber.

The two-cycle combustion engine according to a third aspect of thepresent invention includes a first scavenging passage for communicatingdirectly between a combustion chamber and a crank chamber, a secondscavenging passage for communicating between the combustion chamber andthe crank chamber through a bearing for a crankshaft, an air passage forintroducing an air into the second scavenging passage, a reed valvedisposed in the air passage, and an air-fuel mixture passage forintroducing an air-fuel mixture into the crank chamber, and is sodesigned that during an intake stroke of the engine, the air from theair passage is introduced into the second scavenging passage through thereed valve and the air-fuel mixture from the air-fuel mixture passage isintroduced into the crank chamber, and that during a scavenging strokeof the engine, introduction of the air within the second scavengingpassage into the combustion chamber takes place before the air-fuelmixture within the crank chamber is introduced into the combustionchamber through the first scavenging passage.

This two-cycle combustion engine is featured in that in place of thesuction chamber defined in the side face of the piston according to thesecond aspect of the present invention, the reed valve is employed inthe air passage, and except for this difference, other basic structuralfeatures thereof remain the same. With this two-cycle combustion engine,since when a portion of the air-fuel mixture introduced into the crankchamber enters the second scavenging passage during the scavengingstroke, this air-fuel mixture flows through the bearing for thecrankshaft, the bearing for the crankshaft can be satisfactorilylubricated with a simple structure. Also, since during the scavengingstroke the air introduced into the second scavenging passage during theintake stroke is introduced into the combustion chamber prior to theair-fuel mixture being introduced from the first scavenging passage intothe combustion chamber, the blow-off of the air-fuel mixture can besatisfactorily suppressed by the air so introduced first into thecombustion chamber. Also, the reed valve is opened during the intakestroke to allow the air to be introduced from the air passage into thesecond scavenging passage. In other words, while in the two-cyclecombustion engine according to the second aspect of the presentinvention, no air can be introduced into the second scavenging passagewhen during the intake stroke the cylinder block closes the suctionchamber in the piston, the two-cycle combustion engine according to thisthird aspect of the present invention is such that the air is introducedat all times during a period in which the reed valve is opened in theintake stroke during which a negative pressure is developed inside thecrank chamber, a sufficient amount of the air can be secured within thesecond scavenging passage.

In the two-cycle combustion engine according to a preferred embodimentof the present invention, the two-cycle combustion engine according tothe first aspect thereof is additionally provided with a thirdscavenging passage, which is positioned at a location closer to anexhaust port opening to the combustion chamber for discharging anexhaust gas from the combustion chamber than the second scavengingpassage, and is featured in that during the scavenging stroke,introduction of the air within the crank chamber into the combustionchamber through the second scavenging passage takes place before anair-fuel mixture introducing timing, at which the air-fuel mixturewithin the first scavenging passage is introduced into the combustionchamber, and that simultaneously with the air-fuel mixture introducingtiming or at a timing thereafter, introduction of the air within thecrank chamber into the combustion chamber through the third scavengingpassage takes place, and except for those differences, other basicstructural features thereof remain the same. With this two-cyclecombustion engine, as is the case with the two-combustion engineaccording to the first aspect of the present invention, the bearing forthe crankshaft can be lubricated with a simple structure while theblow-off of the air-fuel mixture is suppressed. Also, since introductionof the air within the crank chamber into the combustion chamber throughthe second scavenging passage takes place before the air-fuel mixtureintroducing timing, at which the air-fuel mixture within the firstscavenging passage is introduced into the combustion chamber, and,simultaneously with the air-fuel mixture introducing timing or at thetiming thereafter, the air within the crank chamber is introduced into aregion of the combustion chamber adjacent the exhaust port, the blow-offof the air-fuel mixture can be more satisfactorily suppressed.

In another preferred embodiment of the present invention, in thetwo-cycle combustion engine according to the first aspect of the presentinvention, the piston has a lubricant passage formed therein forsupplying the air-fuel mixture within the suction chamber to a small endbearing disposed between a piston pin and a connecting rod. According tothis structural feature, the small end bearing is lubricated by theutilization of the air-fuel mixture introduced into the suction chamber.

The two-cycle combustion chamber according to a still further preferredembodiment of the present invention is featured in that in the two-cyclecombustion engine according to the second aspect of the presentinvention, an air regulating valve is provided for closing the airpassage when a pressure inside the air passage decreases to a valueequal to or lower than a predetermined value, and except for thisdifference, other basic structural features thereof remain the same.Even in the case of this two-cycle combustion engine, as is the casewith the two-cycle combustion engine according to the second aspect ofthe present invention, the bearing for the crankshaft can be lubricatedwith a simple structure while the blow-off of the air-fuel mixture issuppressed. Also, during a high boosting such as, for example, anidling, that is, when the pressure inside the air passage decreases tothe value equal to or lower than the predetermined value, the airpassage is closed by the air regulating valve and, therefore,introduction of the air into the crank chamber is interrupted. For thisreason, dilution of the air-fuel mixture being introduced from the crankchamber to the combustion chamber during the idling can be avoided tothereby stabilize the rotation of the two-cycle combustion engine.

In a still further preferred embodiment of the present invention, in thetwo-cycle combustion engine according to the second or third aspect ofthe present invention, an opening of the first scavenging passagetowards the crank chamber is closed by the piston before the pistonreaches a bottom dead center. According to this feature, since when thepiston nears the bottom dead center, the first scavenging passage isclosed, introduction of the air-fuel mixture within the crank chamberinto the combustion chamber at the end of the scavenging stroke can beprevented. For this reason, the blow-off of the air-fuel mixture can bemore satisfactorily suppressed.

In a still further preferred embodiment of the present invention, in thepreferred embodiment in which the second and third scavenging passagesare employed, an opening of the second scavenging passage towards thecrank chamber is closed by the piston before the piston reaches a bottomdead center. According to this feature, since the internal pressureinside the crank chamber increases as the piston approaches the bottomdead center, closure of the second scavenging passage with the piston ata location in the vicinity of the bottom dead center is effective toincrease the force of blow-off of the air from the third scavengingpassage which opens at a location adjacent the exhaust port. For thisreason, the blow-off of the air-fuel mixture can be more satisfactorilysuppressed.

In a still further preferred embodiment of the present invention, thesecond scavenging passage is positioned at a location closer to anexhaust port than the first scavenging passage in a directioncircumferentially of the combustion chamber. According to this feature,since the air from the second scavenging passage can be supplied to aregion in the combustion chamber adjacent the exhaust port, the blow-offof the air-fuel mixture from this exhaust port can be moresatisfactorily suppressed.

The two-cycle combustion engine according to a fourth aspect of thepresent invention includes a first scavenging passage for communicatingdirectly between a combustion chamber and a crank chamber, a secondscavenging passage for communicating between the combustion chamber andthe crank chamber through a bearing for a crankshaft, an air-fuelmixture passage for introducing an air-fuel mixture into the firstscavenging passage, an air passage for introducing an air into thesecond scavenging passage, a first reed valve disposed in the air-fuelmixture passage, and a second reed valve disposed in the air passage,and is so designed in that during an intake stroke of the engine, theair-fuel mixture from the air-fuel mixture passage is introduced intothe first scavenging passage and the air from the air passage isintroduced into the second scavenging passage, and that during ascavenging stroke of the engine, introduction of the air within thesecond scavenging passage into the combustion chamber takes place beforethe air-fuel mixture within the first scavenging passage is introducedinto the combustion chamber.

This two-cycle combustion engine is featured in that in the two-cyclecombustion engine according to the third aspect of the present inventionhaving the reed valve in the air passage, the first reed valve isemployed in the air-fuel mixture passage, and except for thisdifference, other basic structural features thereof remain the same.With this two-cycle combustion engine, during the intake stroke theair-fuel mixture from the air-fuel mixture passage can be onceintroduced into the first scavenging passage through the first reedvalve and the air from the air passage can be once introduced into thesecond scavenging passage through the second reed valve. Accordingly,only principally necessary amounts of the air-fuel mixture and the aircan be allowed to fill up the first and second scavenging passages,respectively. For this reason, it is possible to prevent, an excessivelyenriched air-fuel mixture from entering the combustion chamber at theend of the scavenging stroke and then blowing off through the exhaustport. Also, a portion of the air-fuel mixture introduced into the firstscavenging passage enters the crank chamber and lubricates the bearingfor the crankshaft when entering the second scavenging passage duringthe scavenging stroke. In addition, since the air-fuel mixture behavesin such a manner that the enriched air-fuel mixture within the firstscavenging passage enters the combustion chamber and, thereafter, theair-fuel mixture within the crank chamber, which is a leaned air-fuelmixture, enters the combustion chamber through the first scavengingpassage, the blow-off of the enriched air-fuel mixture can be avoidedwith the charging efficiency increased consequently.

The two-cycle combustion engine according to a fifth aspect of thepresent invention includes a needle bearing for supporting a crankshaftwithin a crankcase, first and second scavenging passages forcommunicating between a combustion chamber and a crank chamber, anair-fuel mixture passage for introducing an air-fuel mixture into thecrank chamber or the first scavenging passage during an intake stroke,an air passage for introducing an air into the second scavenging passageor the crank chamber during the intake stroke, and a communicating holefor fluidly connecting the first or second scavenging passage with theneedle bearing, and is so designed that during a scavenging stroke,introduction of the air within the second scavenging passage into thecombustion chamber takes place prior to the air-fuel mixture within thefirst scavenging passage being introduced into the combustion chamber,and that an opening of a lower end of the second scavenging passagetowards the crank chamber is positioned at a location adjacent a regionradially outwardly of the needle bearing.

According to the fifth aspect of the present invention, during thescavenging stroke the air-fuel mixture within the crank chamber flowsinto the needle bearing from the first or second scavenging passagethrough the communicating hole to thereby lubricate the needle bearing.Also, since as compared with ball bearings generally used to support thecrankshaft, the needle bearing has a small outer diameter, extension ofthe second scavenging passage downwardly straight a distancecorresponding to the difference in outer diameter is effective toincrease the capacity so that a sufficient amount of the air can besecured in the second scavenging passage. Accordingly, during thescavenging stroke, the sufficient amount of the air can be injected fromthe second scavenging passage into the combustion chamber. Moreover,since the second scavenging passage can be formed to extend straighthaving an increased length, an undesirable increase of the flowresistance therein can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front sectional view of a two-cycle combustion engineaccording to a first preferred embodiment of the present invention;

FIG. 2 is a side sectional view of the two-cycle combustion engineduring a suction stroke, showing a cylinder block and a crankcase on anenlarged scale and also showing first scavenging passages;

FIG. 3 is a side sectional view of the two-cycle combustion engine ofFIG. 2 during a scavenging stroke, showing the cylinder block and thecrankcase on an enlarged scale and also showing first scavengingpassages employed therein;

FIG. 4 is a side sectional view of the two-cycle combustion engine,showing the cylinder block and the crankcase on an enlarged scale andalso showing second scavenging passages employed therein;

FIG. 5 is a front sectional view of the two-cycle combustion engine,showing the cylinder block and the crankcase on an enlarged scale;

FIG. 6 is a front sectional view, showing the relation in heightwiseposition between an exhaust port in the cylinder block and the first andsecond scavenging passages employed therein;

FIG. 7 is a side view, showing the appearance of the cylinder block ofthe two-cycle combustion engine;

FIG. 8 is a cross-sectional view taken along the line VIII-VIII in FIG.5;

FIG. 9 is a cross-sectional view taken along the line IX-IX in FIG. 5;

FIG. 10 is a side sectional view of the two-cycle combustion engine inthe suction stroke according to a second preferred embodiment of thepresent invention, showing the second scavenging passages employedtherein;

FIG. 11 is a side sectional view of the two-cycle combustion engine ofFIG. 10 in the scavenging stroke, showing the second scavenging passagesemployed therein;

FIG. 12 is a side sectional view of the two-cycle combustion engine ofFIG. 10 in the scavenging stroke, showing the first scavenging passagesemployed therein;

FIG. 13 is a front sectional view of the two-cycle combustion engine ofFIG. 10, showing the cylinder block and the crankcase employed thereinon an enlarged scale;

FIG. 14 is a side view showing the appearance of the cylinder block ofthe two-cycle combustion engine of FIG. 10;

FIG. 15 is a front elevational view of a piston employed in thetwo-cycle combustion engine of FIG. 10;

FIG. 16 is a cross-sectional view taken along the line XVI-XVI in FIG.13;

FIG. 17 is a cross-sectional view taken along the line XVII-XVII in FIG.13;

FIG. 18 is a front sectional view of the two-cycle combustion engineaccording to a third preferred embodiment of the present invention,showing the cylinder block and the piston employed therein;

FIG. 19 is a cross-sectional view taken along the line XIX-XIX in FIG.18;

FIG. 20A is a front sectional view of the two-cycle combustion engineaccording to a fourth preferred embodiment of the present invention;

FIG. 20B is a front elevational view showing an air regulating valveemployed in the two-cycle combustion engine of FIG. 20A;

FIG. 21 is a front sectional view of the two-cycle combustion engineaccording to a fifth preferred embodiment of the present invention,showing the cylinder block and the crankcase employed therein;

FIG. 22 is a cross-sectional view taken along the line XXII-XXII in FIG.21;

FIG. 23 is a cross-sectional view taken along the line XXIII-XXIII inFIG. 21;

FIG. 24 is a front sectional view of the two-cycle combustion engineaccording to a sixth preferred embodiment of the present invention,showing the cylinder block and the crankcase employed therein;

FIG. 25 is a front elevational view, showing the cylinder block of thetwo-cycle combustion engine of FIG. 24;

FIG. 26 is a cross-sectional view taken along the line XXVI-XXVI in FIG.24;

FIG. 27 is a side sectional view of the two-cycle combustion engine ofFIG. 24, showing the cylinder block and the crankcase and also showingthe first scavenging passages;

FIG. 28 is a side sectional view of the two-cycle combustion engine ofFIG. 24, showing the cylinder block and the crankcase and also showingthe second scavenging passages;

FIG. 29 is a front sectional view of the two-cycle combustion engineaccording to a seventh preferred embodiment of the present invention,showing the cylinder block and the crankcase employed therein;

FIG. 30 is a side sectional view of the two-cycle combustion engine ofFIG. 29, showing the cylinder block and the crankcase employed therein;

FIG. 31 is a front sectional view of the two-cycle combustion engineaccording to an eighth preferred embodiment of the present invention,showing the cylinder block and the crankcase employed therein;

FIG. 32 is a side sectional view of the two-cycle combustion engine ofFIG. 31, showing the cylinder block and the crankcase employed therein;

FIG. 33 is a timing chart showing the sequence of operation of thetwo-cycle combustion engine of FIG. 31;

FIG. 34 is a front sectional view of the two-cycle combustion engineaccording to a ninth preferred embodiment of the present invention,showing the cylinder block and the crankcase employed therein; and

FIG. 35 is a side sectional view of the two-cycle combustion engine ofFIG. 34, showing the cylinder block and the crankcase employed therein.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with particular reference to the accompanyingdrawings.

Referring to FIG. 1 showing a front sectional view of a two-cyclecombustion engine according to a first preferred embodiment of thepresent invention, a cylinder block 1 having a combustion chamber 1 adefined therein is fixedly mounted on an upper portion of a crankcase 2.The cylinder block 1 has a side portion (right side portion) to which acarburetor 3 and an air cleaner 4, both forming respective parts of anintake system of the engine, are fluidly connected in series, and alsohas an opposite side portion (left side portion) to which a muffler 5forming a part of an exhaust system of the engine is fluidly connected.A fuel tank 6 is secured to a bottom portion of the crankcase 2. Thecylinder block 1 is provided with a piston 7 capable of reciprocating inan axial direction (in the embodiment shown, in a vertical direction). Acrankshaft 8 is supported within an interior of the crankcase 2 by meansof bearings 81. A hollow crankpin 82 is provided at a location offsetrelative to a longitudinal axis of the crankshaft 8, and the crankpin 82and a hollow piston pin 71, provided on the piston 7, are connected witheach other by a connecting rod 83. In this figure, reference numeral 84represents crank webs provided on the crankshaft 8. Also, referencecharacter P represents an ignition plug mounted atop the cylinder block1.

An adaptor 9 is provided between the cylinder block 1 and the carburetor3, and an air-fuel mixture passage 10 is formed within the cylinderblock 1 and respective interiors of the carburetor 3 and the adaptor 9.This passage 10 introduces an air-fuel mixture M into suction chambers72 defined in a peripheral wall of the piston 7 when, during an intakestroke, the piston 7 nears the top dead center, as will be describedlater. The air-fuel mixture M introduced into the suction chambers 72 isintroduced through first scavenging passages 13, as will be describedlater, into a crank chamber 2 a defined within the crankcase 2 and belowthe cylinder block 1.

Also, in a region below the air-fuel mixture passage 10, an air passage11 is formed so as to extend parallel thereto, and an air A from thisair passage 11 is introduced from an air port 11 a, opening in an innerperipheral surface of the cylinder block 1, directly into the crankchamber 2 a during the intake stroke. The carburetor 3 is such as toadjust the respective cross-sectional areas of the air-fuel mixturepassage 10 and the air passage 11 by means of a single rotary valve.Also, an exhaust passage 12 having an exhaust port 12 a opening at theinner peripheral surface of the cylinder block 1 is formed in aperipheral wall of the cylinder block 1, and exhaust gases flowingthrough this exhaust passage 12 are exhausted to the outside through themuffler 5.

FIGS. 2 to 4 are side sectional views showing the cylinder block and thecrankcase on an enlarged scale, in which FIGS. 2 and 3 illustrate thefirst scavenging passages 13 and FIG. 4 illustrates second scavengingpassages 14. In each of those figures, the manner of movement of theair-fuel mixture M and the air A according to the position of the pistonis shown, the details of which will be described later. As shown in FIG.2, the first scavenging passages 13 for introducing the air-fuel mixtureM from the air-fuel mixture passage 10 (FIG. 1) are formed within thecylinder block 1 and the crankcase 2. The first scavenging passages 13fluidly connect between the combustion chamber 1 a in the cylinder block1 and the crank chamber 2 a through the bearings 81 for the crankshaft8. In other words, each of the first scavenging passages 13 has a firstscavenging port 13 a, opening in the inner peripheral surface of thecylinder block 1 or a cylinder bore, and a communicating passage 13 bextending vertically from the first scavenging port 13 a over a bottomend of the cylinder block 1 and down to an outer side face of theadjacent bearing 81, which is situated at a location intermediate of theheight of the crankcase 2. During the intake stroke, the air-fuelmixture M introduced from the air-fuel mixture passage 10, shown in FIG.1, into the suction chambers 72 is introduced from the first scavengingports 13 a into the communicating passages 13 b shown in FIG. 2 and is,after having been caused to flow through respective gaps between innerand outer races of ball bearings employed as the bearings 81 for thecrankshaft 8, introduced into the crank chamber 2 a through respectivegaps between the bearings 81 and the crank webs 84 so that the bearings81 can be lubricated with fuel then contained in the air/furl mixture M.On the other hand, even during a scavenging stroke, some amount of theair-fuel mixture entered the crank chamber 2 a is introduced into thefirst scavenging passages 13 through the respective gaps in the bearings81 and is then utilized to lubricate the bearings 81. The air-fuelmixture M is, as shown in FIG. 3, supplied from the first scavengingpassages 13 into the combustion chamber 1 a above the piston 7.

Also, in the embodiment now under discussion, an oiling passage 85 isformed for fluidly connecting between the crank chamber 2 a and thefirst scavenging passages 13 through a hollow of the crank shaft 8 shownin FIG. 2. This oiling passage 85 is made up of first passageways 85 aextending axially and opening into the crank chamber 2 a and secondpassageways 85 b extending radially so as to fluid connect between thefirst passageways 85 a and the first scavenging passages 13,respectively. Yet, a big end bearing (needle bearing) 89 is interposedbetween a big end of the connecting rod 83 and the crankpin 82, and aportion of the crankshaft 8 in the vicinity of the crankpin 82 is formedwith a plurality of circumferentially spaced communicating holes 88 soas to extend axially for fluidly connecting between the big end bearing89 and the left and right bearings 81. By so doing, the big end bearing89 can also be lubricated with the air-fuel mixture flowing through thefirst scavenging passages 13. In addition, a sliding interface betweenside faces of the big end of the connecting rod 83 and the crankshaft 8can also be lubricated with the air-fuel mixture M supplied from thefirst scavenging passages 13 by way of the oiling passage 85.

Furthermore, a lubricating passage 73 is formed for supplying a portionof the air-fuel mixture M within the suction chambers 72 in the piston 7to a small end bearing 90. As shown in FIG. 2, this lubricating passage73 is made up of an axially extending lubricating groove 73 a formed inthe piston 7 at a location adjacent an outer periphery of the piston pin71, and a lubricating hole 73 b for communicating the suction chambers72 with the lubricating groove 73 a With this design, the small endbearing 90 can be lubricated with a portion of the air-fuel mixtureintroduced into the suction chambers 72 during the intake stroke.

As shown in FIG. 4, the second scavenging passages 14 for the flow ofair, which are fluidly connected between the combustion chamber 1 a andthe crank chamber 2 a, are formed in the cylinder block and thecrankcase 2. Each of the second scavenging passages 14 has a secondscavenging port 14 a, opening in the inner peripheral surface of thecylinder block 1 or the cylinder bore, and a communicating passage 14 bextending vertically from the second scavenging port 14 a over thebottom end of the cylinder block 1 and opening in an inner peripheralsurface of the upper portion of the crankcase 2. The air A introducedfrom the air port 11 a into the crank chamber 2 a is, during thescavenging stroke, injected into the combustion chamber 1 a through thescavenging ports 14 a by way of the communicating passages 14 b.

FIG. 5 illustrates a front sectional view with the cylinder block 1 andthe crankcase 2 shown on an enlarged scale. As shown therein, the firstand second scavenging passages 13 and 14 are employed in a pair,respectively, and extend parallel to each other in the verticaldirection, in which the second scavenging port 14 a defined at an upperend of the second scavenging passage 14 has its upper edge positioned ata level lower than an upper edge of the exhaust port 12 a. Also, thefirst scavenging port 13 a defined at an upper end of the firstscavenging passage 13 has its upper edge positioned at a level lowerthan the upper edge of the adjacent second scavenging port 14 a.

FIG. 6 illustrates a diagram showing the relation in heightwise positionbetween the exhaust port 12 a and the first and second scavenging ports13 a and 14 a. As shown therein, when respective positions of the upperedges of the exhaust port 12 a, the second scavenging ports 14 a and thefirst scavenging ports 13 a are expressed by H1, H2 and H3, H1 assumesthe highest position, followed by H2 which is in turn followed by H3, ina direction from top. Accordingly, during the scavenging stroke, the airA from the second scavenging ports 14 a can be injected prior to theair-fuel mixture M from the first scavenging ports 13 a.

FIG. 7 illustrates a side view showing the appearance of the cylinderblock 1. The cylinder block 1 has an outer side portion formed with acutout 10 a of a generally inverted V-shape, which forms a part of thedownstream region of the air-fuel mixture passage 10. Two air-fuelmixture introducing ports 10 b and 10 b capable of opening into therespective suction chambers 72 (FIG. 2) formed in the peripheral wall ofthe piston 7, when the piston 7 nears the top dead center, are providedin deep inner regions of opposite sides of this cutout 10 a. Also, at aposition below the cutout 10 a, a cutout or a hole 11 b forming a partof the air passage 11 is formed, and the air port 11 a (FIG. 6) openingin the inner peripheral surface of the cylinder block 1 is formed in adeep inner region thereof.

FIG. 8 is a cross-sectional view taken along the line VIII-VIII in FIG.5 and FIG. 9 is a cross-sectional view taken along the line IX-IX inFIG. 5. As shown in FIG. 8, the piston 7 is formed with the pair of thesuction chambers 72 formed by depressing respective opposite portions ofthe peripheral wall of the piston 7 radially inwardly. When during theintake stroke the piston 7 nears the top dead center with the ports 10 bin the cutout 10 a of the air-fuel mixture passage 10 aligned with therespective suction chambers 72, the air-fuel mixture M is introducedfrom the ports 10 b into the suction chambers 72 and then through thesuction chambers 72 into the crank chamber 2 a by way of the firstscavenging ports 13 a and the communicating passages 13 b of the firstscavenging passages 13, shown in FIG. 2, and the communicating passages13 b, respectively. Also, during the scavenging passage in which thepiston 7 descends, as shown in FIG. 9, the combustion chamber 1 a isscavenged with the air A, injected from the second scavenging ports 14a, and the air-fuel mixture M injected from the first scavenging ports13 a following the injection of the air A.

Hereinafter, the operation of the two-cycle combustion engine of thestructure so designed as hereinbefore will be described.

In the first place, when, during the intake stroke, the piston 7 withinthe cylinder block 1 nears the top dead center as shown in FIG. 2, thepair of the suction chambers 72 defined in the peripheral wall of thepiston 7 are communicated with the air-fuel mixture introducing ports 10b of the air-fuel mixture passage 10 provided in the cylinder block 1.Also, since during this intake stroke a negative pressure is developedinside the crank chamber 2 a as a result of the ascending motion of thepiston 7, the air-fuel mixture M introduced from the ports 10 b into therespective suction chambers 72 can be introduced into the firstscavenging passages 13 through the first scavenging ports 13 a and,then, a portion thereof is introduced into the crank chamber 2 a throughthe communicating passages 13 b and the bearings 81 for the crankshaft8. Accordingly, the bearings 81 can be sufficiently lubricated with asimplified structure, by the fuel contained in the air-fuel mixture Mflowing through the bearings 81.

Also, during the intake stroke, as shown in FIG. 2, the air A flowing inthe air passage 11 is introduced into the crank chamber 2 a through theair port 11 a opening in the inner peripheral surface of the cylinderblock 1.

Subsequently and during the scavenging stroke, in which the piston 7descends towards the bottom dead center, as shown in FIGS. 3 and 4, theair-fuel mixture M and the air A are injected into the combustionchamber 1 a from the first and second scavenging ports 13 a and 14 a ofthe first and second scavenging passages 13 and 14, respectively. Atthis time, since as shown in FIG. 6, the respective positions H1, H2 andH3 of the upper edges of the exhaust port 12 a, the second scavengingports 14 a and the first scavenging ports 13 a are so chosen as to lowerin the specific order with H1 assuming the highest position, during thescavenging stroke the air A can be first injected through the secondscavenging ports 14 a as shown by the arrows in FIG. 9, followed byinjection of the air-fuel mixture M through the first scavenging ports13 a. Also, the air A is injected at a location closer to the exhaustport 12 a than the location at which the air-fuel mixture M is injected.For this reason, a blow-off of the air-fuel mixture M from the exhaustport 12 a can be suppressed by the air A first introduced into thecombustion chamber 1 a. Even during this scavenging stroke, the bearings81 can be lubricated when the air-fuel mixture M somewhat entering thecrank chamber 2 a shown in FIG. 2 returns to the first scavengingpassages 13 through the bearings 81.

In the next place, the two-cycle combustion engine according to a secondpreferred embodiment of the present invention will be described. Thistwo-cycle combustion engine according to the second embodiment differsfrom that according to the first embodiment in that the paths of theair-fuel mixture and the air, respectively, are reversed relative toeach other. In other words, except that in the two-cycle combustionengine according to the second embodiment, during the intake stroke theair-fuel mixture M is introduced directly into the crank chamber throughthe air-fuel mixture passage and on the other hand, the air A isintroduced into the second scavenging passages through the air passage,other structural features of the two-cycle combustion engine accordingto the second embodiment are similar to those according to the firstembodiment. FIGS. 10 to 12 illustrate side sectional views of thetwo-cycle combustion engine with the cylinder block and the crankcaseshown on an enlarged scale, in which FIGS. 10 and 11 illustrateparticularly second scavenging passages 22 and FIG. 12 illustratesparticularly first scavenging passages 21. In each of those figures, themanner of movement of the air-fuel mixture M and the air A according tothe position of the piston is shown, the details of which will bedescribed later.

In this two-cycle combustion engine, as shown in FIG. 12, within thecylinder block 1 and the crankcase 2, the first scavenging passages 21for fluidly connecting directly between the combustion chamber 1 a andthe crank chamber 2 a are provided and, also, as shown in FIG. 10, thesecond scavenging passages 22 for fluidly connecting between thecombustion chamber 1 a and the crank chamber 2 a through the bearings 81for the crankshaft 8 are provided. As shown in FIG. 13, first and secondports 21 a and 22 a provided in those first and second scavengingpassages 21 and 22, respectively, are so positioned that as is the casewith the previously described two-cycle combustion engine, the upperedge of each of the second scavenging ports 22 a can occupy a positionhigher than the upper edge of each of the first scavenging ports 21 aand lower than the exhaust port 12 a.

Each of the first scavenging passages 21 shown in FIG. 12 has the firstscavenging port 21 a opening in the inner peripheral surface of thecylinder block 1, a communicating passage 21 b extending vertically fromthe first scavenging port 21 a over the bottom end of the cylinder block1 down to the upper portion of the crankcase 2, and an inflow port 21 copening in the inner peripheral surface of the upper portion of thecrankcase 2. The air-fuel mixture M introduced into the crank chamber 2a is, during the scavenging stroke, injected into the combustion chamber1 a from the scavenging ports 21 a through the communicating passages 21b. The air-fuel mixture M is, during the intake stroke, introduceddirectly into the crank chamber 2 a from an air-fuel mixture port 20,communicated with the air-fuel mixture passage 10 (FIG. 13) and openingin the inner peripheral surface of the cylinder block 1 as shown in FIG.13, as shown by the arrows.

When the piston 7 descends down to a position near the bottom deadcenter, the inflow ports 21 c shown in FIG. 12 are closed by theperipheral wall of the piston 7 to shut the first scavenging passages 21to thereby prevent the air-fuel mixture M within the crank chamber 2 afrom entering the combustion chamber 1 a through the first scavengingpassages 21. Accordingly, introduction of the air-fuel mixture M withinthe crank chamber 2 a into the combustion chamber 1 a at the end of thescavenging stroke can be interrupted and, therefore, the blow-off can befurther effectively suppressed.

Also, as shown in FIG. 10, each of the second scavenging passages 22 hasa second scavenging port 22 a, opening in the inner peripheral surfaceof the cylinder block 1, and a communicating passage 22 b extendingvertically from the second scavenging port 22 a over the bottom end ofthe cylinder block 1 and down to the outer side face of the adjacentbearing 81, which is situated at a location intermediate of the heightof the crankcase 2. The air A introduced from the air passage 11 (FIG.13) into the second scavenging passages 22 is, during the scavengingstroke, injected into the combustion chamber 1 a from the scavengingports 22 a through the communicating passages 22 b as shown in FIG. 11.

FIG. 14 illustrates a side view showing the appearance of the cylinderblock 1. As shown therein, the cylinder block 1 has an outer sideportion formed with a cutout 11 b of a generally inverted V-shape, whichforms a part of the air passage 11, and two air introducing ports 11 cand 11 c capable of opening into the respective suction chambers 72A(FIG. 10) formed in the peripheral wall of the piston 7, when the piston7 nears the top dead center, are provided in deep inner regions ofopposite sides of this cutout 11 b. Also, at a position below the cutout11 b, an air-fuel port 20 communicated with the air-fuel passage 10 andopening in the inner peripheral surface of the cylinder block 1 isformed.

FIG. 15 is a front elevational view showing the piston. As showntherein, lower portions of the peripheral wall of the piston 7 areformed with generally L-shaped suction chambers 72A, respectively, eachmade up of a rectangular recess 72 a and an elongated groove 72 bextending from the recess 72 a in a direction circumferentially of thepiston 7.

FIG. 16 is a cross-sectional view taken along the line XVI-XVI in FIG.13, and FIG. 17 is a cross-sectional view taken along the line XVII-XVIIin FIG. 13. As shown in FIG. 16, when the piston 7 nears the top deadcenter, respective portions of the grooves 72 b of the suction chambers72A are aligned with the ports 11 c of the cutout 11 b so that the air Aintroduced into the cutout 11 b can be introduced from the ports 11 c tothe second scavenging ports 22 a of the second scavenging passages 22through the recesses 72 a of the associated suction chambers 72 as shownby the arrows and be further introduced therefrom into the interiors ofthe second scavenging passages 22. Also, during the scavenging stroke inwhich the piston 7 descends, as shown in FIG. 17, the combustion chamber1 a is scavenged by the air A, injected through the second scavengingports 22 a, and the air-fuel mixture M subsequently injected through thefirst scavenging ports 21 a following the air A.

The operation of the two-cycle combustion engine of the structure sodesigned as hereinbefore will be described.

In the first place, when, during the intake stroke, the piston 7 withinthe cylinder block 1 nears the top dead center as shown in FIG. 10, theair-fuel mixture M are introduced directly from the air-fuel mixtureport 20, opening in the inner peripheral surface of the cylinder block1, into the crank chamber 2 a. With this air-fuel mixture M sointroduced, the bearings 81 for the crankshaft 8 and the crankpin 82 canbe satisfactorily lubricated with a simple structure as is the case withthe previously described first embodiment.

Also, during the intake stroke, the suction chambers 72A provided in thepiston 7 are communicated with the air introducing ports 11 c of the airpassage 11 provided in the cylinder block 1. Accordingly, by the effectof a negative pressure inside the crank chamber 1 a, the air Aintroduced into the cutouts 11 b is further introduced into the secondscavenging passages 22 and the crank chamber 2 a through the secondscavenging ports 22 a.

Subsequently and during the scavenging stroke, as shown in FIG. 17, theair-fuel mixture M and the air A are injected into the combustionchamber 1 a from the first and second scavenging ports 21 a and 22 a ofthe first and second scavenging passages 21 and 22, respectively.Specifically, the air A is first injected from the second scavengingports 22 a and, thereafter, the air-fuel mixture M is injected from thefirst scavenging ports 21 a in a manner delayed relative to the air A.By the effect of the air so introduced prior to the air-fuel mixture M,the blow-off of the air-fuel mixture M through the exhaust port 12 a canbe suppressed. When the air A is injected into the combustion chamber 1a through the second scavenging passages 22 shown in FIG. 11, a portionof the air-fuel mixture M within the crank chamber 2 a flows into thesecond scavenging passages 22 through the gaps between the inner andouter races of the respective bearings 81. In this way, the bearings 81can be lubricated with the fuel contained in the air-fuel mixture.

The two-cycle combustion engine according to a third preferredembodiment of the present invention will now be described. The two-cyclecombustion engine according to the third embodiment is similar to thataccording to the previously described second embodiment, but is featuredin that in place of the suction chambers 72A defined in the peripheralwall of the piston 7, a reed valve is employed for closing the airpassage when the pressure inside the air passage decreases to a valueequal to or lower than a predetermined value, and other structuralfeatures of the two-cycle combustion engine according to the thirdembodiment are similar to those according to the second embodiment.

FIG. 18 is a front sectional view of the two-cycle combustion engineaccording to the third preferred embodiment of the present invention,showing the cylinder block and the piston employed therein, and FIG. 19is a cross-sectional view taken along the line XIX-XIX in FIG. 18. Asshown in FIG. 18, the piston 7 is not provided with any suction chamber.As shown in FIG. 19, opposite side portions of the cutout 11 b (FIG. 18)for the air in the cylinder block 1 are provided with respective airintroducing ports 11 d and 11 d, and respective outer walls of thesecond scavenging passages 22 are provided with two air discharge ports11 e and 11 e, with the neighboring air introducing and discharge ports11 d and 11 e being fluidly connected with each other by means ofrespective connecting pipes 30. Also, an adaptor 31 having the airpassage 11 defined therein and communicated with the carburetor 3 isfitted to an outer side portion of the cutout 11 b, and a reed valve 32for closing the air passage 11 when the pressure inside the air passage11 decreases down to a value equal to or lower than a predeterminedvalue is fitted to a portion of the interior of the adaptor 31 whichconfronts the cutout 11 b.

According to the third embodiment described above, when, during theintake stroke, a negative pressure is developed inside the cylinderblock 1 and the crank chamber 2 a shown in FIG. 10, the reed valve 32shown in FIG. 18 is opened to allow the air A flowing in the air passage11 to be introduced into the crank chamber 2 a through the cutout 11 b,then through the connecting pipes 30 (FIG. 19) and finally through theassociated second scavenging passage 22. Accordingly, while in thetwo-cycle combustion engine according to the second embodiment, no airis introduced into the second scavenging passages 22 when the suctionchambers 72A in the piston 7 leave away from the scavenging ports 22 aof the second scavenging passage 22 (FIG. 10), the two-cycle combustionengine according to this third embodiment is such that when the reedvalve 32 shown in FIG. 18 opens in response to the negative pressureinside the crank chamber 2 a during the intake stroke, the air A isintroduced into the second scavenging passages 22 at all times. For thisreason, a sufficient amount of air necessary for the prevention of theblow-off can be secured within the second scavenging passages 22. Also,since no suction chamber 72A is necessary in the piston 7, the passagestructure for the introduction of the air can be simplified and thepiston 7 can be manufactured light-weight.

Further, the two-cycle combustion engine according to a fourth preferredembodiment of the present invention will now be described. Thistwo-cycle combustion engine is featured in that in the two-cyclecombustion engine according to the previously described secondembodiment, an air regulating valve capable of closing the air passagewhen the pressure inside the air passage decreases down to a value equalto or lower than a predetermined value is employed, and other structuralfeatures of the two-cycle combustion engine according to the fourthembodiment are similar to those according to the second embodiment.

FIG. 20A is a front elevational view showing, with a portion cut out,the two-cycle combustion engine according to the fourth embodiment. Inthe two-cycle combustion engine shown therein, the cylinder block 1 hasan outer side to which an adaptor 40 having an air-fuel mixture passage10 defined therein in communication with the carburetor 3 is fitted, andan air introducing passage 41 having an inner end communicated with thecutout 11 b forming the air passage 11, provided in the cylinder block1, and an outer end opening towards the atmosphere through an air filter45 is formed above the air-fuel mixture passage 10 in the adaptor 40. Anair regulating valve 44 is provided within the interior of this airintroducing passage 41.

The air regulating valve 44 includes a petal-shaped valve body 42 and acoil spring 43 and is so designed that when the pressure inside the airpassage 11, which receives a negative pressure inside the crank chamber1 a, exceeds a predetermined value or the absolute value of the negativepressure lowers than the absolute value of such predetermined value, thevalve body 42 is urged against a stopper 47 by the spring force of thespring 43 to open an outer peripheral portion of the valve body 42 asshown in FIG. 20B so that the air A from the air filter 45 shown in FIG.20A can be introduced into the air introducing passage 41, the airpassage 11, the suction chambers 72A and then the second scavengingpassages 22. On the other hand, when the pressure inside the air passage11 shown in FIG. 20A decreases down to a value lower than thepredetermined value, the valve body 42 is urged against a valve seat 48against the pressing force of the spring 43 by the action of theatmospheric pressure acting from right of the valve body 42 to therebyclose the valve with the introduction of the air into the secondscavenging passages 22 halted consequently.

It is not generally considered desirable to introduce a large amount ofair into the combustion chamber 1 a during a high boosting such as, forexample, an idling, since the amount of the air-fuel mixture within thecrank chamber 2 a generally decreases. With this fourth two-cyclecombustion engine, during the high boosting, that is, when the pressureinside the air passage 11 decreases down to a value equal to or lowerthan the predetermined value, the air passage 11 is closed by the airregulating valve 44 and, therefore, introduction of the air A into thesecond scavenging passages 22 is interrupted. For this reason, dilutionof the air-fuel mixture within the combustion chamber 1 a during thehigh boosting such as the idling can be avoided to thereby stabilize therotation of the two-cycle combustion engine.

The two-cycle combustion engine according to a fifth preferredembodiment of the present invention will now be described. Thistwo-cycle combustion engine is similar to that according to thepreviously described first embodiment, but is featured in that pairs ofsecond and third scavenging passages having different injectingpositions are employed for communicating between the combustion chamberand the crank chamber, and other structural features of the two-cyclecombustion engine according to the third embodiment than that mentionedabove are similar to those according to the first embodiment.

FIG. 21 is a front sectional view showing the cylinder block and thecrankcase employed therein, FIG. 22 is a cross-sectional view takenalong the line XXII-XXII in FIG. 21 and FIG. 23 is a cross-sectionalview taken along the line XXIII-XXIII in FIG. 21. In the two-cyclecombustion engine shown in FIG. 21, the cylinder block 1 is formed withthe first scavenging passages 13 for fluidly connecting between thecombustion chamber 1 a and the crank chamber 2 a through the bearings 81for the crankshaft 8, and respective pairs of second and thirdscavenging passages 14 and 15 for fluidly connecting directly betweenthe combustion chamber 1 a and the crank chamber 2 a.

The first to third scavenging passages 13 to 15 extend substantiallyvertically in parallel relation to each other and are, as shown in FIGS.22 and 23, employed in a pair. A second scavenging port 14 a provided atan upper end of each of the second scavenging passages 14 has an upperedge thereof defined at a position lower than the upper edge of theexhaust port 12 a, and a first scavenging port 13 a provided at an upperend of each of the first scavenging passages 13 has an upper edgethereof defined at a position lower than the upper edge of the secondscavenging port 14 a. Also, a third scavenging port 15 a provided at anupper end of each of the third scavenging passage 15 has an upper edgethereof defined at a position lower than the upper edge of the secondscavenging port 14 a, but at a position level with or slightly lowerthan the upper edge of the first scavenging port 13 a.

As shown in FIG. 22, the air-fuel mixture M from the air-fuel mixturepassage 10 is introduced from the suction chambers 72, formed in thepiston 7, into the first scavenging passages 13. Also, as shown in FIG.23, the first to third scavenging ports 13 a to 15 a of the respectivefirst to third scavenging passages 13 to 15 are formed in the specificorder from a position adjacent the air-fuel mixture passage 10 towards aposition adjacent the exhaust port 12 a of the exhaust passage 12, withthe third scavenging ports 15 a of the third scavenging passages 15opening in the vicinity of the exhaust port 12 a. In addition, the thirdscavenging ports 15 a are so opened that the air A can be injected inthe vicinity of the exhaust port 12 a in a direction perpendicular to apassage center line of the exhaust port 12, whereas the first and secondscavenging ports 13 a and 14 a are so opened as to inject the air-fuelmixture M and the air A in a direction towards the combustion chamber 1a opposite to the exhaust port 12 a, respectively.

With the fifth two-cycle combustion engine, since prior to the air-fuelmixture M within the first scavenging passages 13 being introduced fromthe first scavenging ports 13 a into the combustion chamber 1 a, the airA within the crank chamber 2 a is injected from the second scavengingports 14 a of the second scavenging passages 14 into the combustionchamber 1 a and, at the same time as the start of injection of theair-fuel mixture M or thereafter, the air A is injected from the thirdscavenging ports 15 a of the third scavenging passages 15 into thecombustion chamber 1 a, the blow-off of the air-fuel mixture can beeffectively prevented by the air A fed from the second and thirdscavenging ports 14 a and 15 a. In particular, since the thirdscavenging ports 15 a of the third scavenging passages 15 are opened inthe vicinity of the exhaust port 12 a and since the air A from the thirdscavenging ports 15 a is injected into the vicinity of the exhaust port12 a in a direction perpendicular to the passage center line of theexhaust port 12 a to thereby interrupt the flow of the air-fuel mixtureM towards the exhaust port 12 a, the blow-off can be further effectivelyprevented.

Also, in the embodiment of FIG. 21, air ports 14 b of the secondscavenging passages 14 and air inflow ports 15 b of the third scavengingpassages 15 are formed in a lower portion of the cylinder block 1. Theair inflow ports 14 b of the second scavenging passages 14 are closed bythe piston 7 when the piston 7 nears the bottom dead center. On theother hand, a lower portion of the piston 7 is formed with cutoutgrooves 7 b which open the air inflow ports 15 b of the third scavengingpassages 15, respectively, when the piston 7 nears the bottom deadcenter.

According to the foregoing construction, when the piston nears thebottom dead center, the air inflow port 14 b, that is, the secondscavenging passages 14 are closed and, on the other hand, in thepresence of the cutout grooves 7 b, the crank chamber 2 a and thecombustion chamber 1 a are maintained in communication with each otherwithout the third scavenging passages 15 being closed. In other words,since, as the piston 7 approaches the bottom dead center, the pressureinside the crank chamber 2 a increases, closure of the second scavengingpassages 14 with the piston 7 then nearing the bottom dead center iseffective to increase the force of injection of the air from the thirdscavenging ports 15 a of the third scavenging passages 15 that open atrespective locations adjacent the exhaust port 12 a. For this reason, atthe later timing at which the amount of the air-fuel mixture M enteringthe combustion chamber 1 a increases, the flow of the air-fuel mixture Mtowards the exhaust port 12 a can be blocked and, therefore, theblow-off of the air-fuel mixture M can be further effectively suppressedsatisfactorily.

Also, in the embodiment of FIG. 21, as is the case with the embodimentshown in FIG. 2, the piston 7 is formed with a lubricating passage 73 soas to extend from each of the suction chambers 72 to the piston pin 71,so that the fuel contained in the air-fuel mixture M introduced into thesuction chambers 72 can be used to lubricate the small end bearing 90 ofthe piston pin 71.

The two-cycle combustion engine according to a sixth preferredembodiment of the present invention will also be described. Thistwo-cycle combustion engine is similar to that according to the thirdembodiment, but is featured in that a first reed valve is employed inthe air-fuel passage and a second reed valve is employed in the airpassage, and other structural features of the two-cycle combustionengine according to the fifth embodiment are similar to those accordingto the third embodiment.

FIG. 24 illustrates a front sectional view showing the cylinder blockand the crankcase of the two-cycle combustion engine and FIG. 25illustrates a front elevational view of the cylinder block. In thetwo-cycle combustion engine shown in FIG. 24, first and second cutouts 1d and 1 e are formed in an outer side face of the cylinder block 1, andan adaptor 60 having first and second passages 61 and 62 formingrespective parts of the air-fuel mixture passage 10 and the air passage11 in cooperation with the cutouts 1 d and 1 e is secured to an outsideof the cylinder block 1. The carburetor 3 is mounted at a locationupstream (right side) of the adaptor 60.

In a region between this adaptor 60 and the cylinder block 1, the firstreed valve 63 that opens during the intake stroke is provided betweenthe first cutout 1 d and the first passage 61, forming a part of theair-fuel mixture passage 10, and the second reed valve 64 that opensduring the intake stroke is provided between the second cutout 1 e andthe second passage 62, forming a part of the air passage 11.

Also, as shown in FIG. 25, opposite side walls of the first cutout 1 din the cylinder block 1 are formed with respective air-fuel mixtureintroducing ports aa and aa in opposition to each other and oppositeside walls of the second cutout 1 e in the cylinder block 1 are formedwith respective air introducing ports bb and bb in opposition to eachother.

FIG. 26 is a cross-sectional view taken along the line XXVI-XXVI in FIG.24. As shown therein, air-fuel mixture discharge ports cc and cc areformed in outer side walls of the first scavenging passages 21,respectively, and are fluidly connected with the air-fuel mixtureintroducing ports aa by means of first connecting pipes 65,respectively. Similarly, air discharge ports dd and dd are formed inouter side walls of the second scavenging passages 22, respectively, andare fluidly connected with the air introducing ports bb by means ofsecond connecting pipes 66, respectively.

FIGS. 27 and 28 are side sectional views showing the cylinder block andthe crankcase, in which FIG. 27 illustrates the first scavengingpassages 21 and FIG. 28 illustrates the second scavenging passages 22.The air-fuel mixture M introduced from the air-fuel mixture passage 10,shown in FIG. 24, by way of the first reed valve 63 is introduced intothe first scavenging passages 21 through the first connecting pipes 65,shown in FIG. 27, and the associated air-fuel mixture discharge ports ccin the cylinder block 1. Also, the air A introduced from the air passage11, shown in FIG. 24, by way of the second reed valve 64 is introducedinto the second scavenging passages 22 through the second connectingpipes 66, shown in FIG. 28, and the associated air discharge ports dd inthe cylinder block 1.

According to the foregoing construction, during the intake stroke inwhich the negative pressure is developed within the crank chamber 2 ashown in FIG. 24, the first reed valve 63 in the air-fuel mixturepassage 10 is opened with the air-fuel mixture M consequently introducedinto the first cutout 1 d through the first passage 61 in the adaptor 60and then into the first scavenging passages 21 through the associatedfirst connecting pipes 65 shown in FIG. 27. A portion of the air-fuelmixture M so introduced into the first scavenging passages 21 enters thecrank chamber 2 a from inflow ports 21 e. The second scavenging passages22 shown in FIG. 28 are communicated with the crank chamber 2 a throughrespective gaps between inner and outer races of the bearings 81.Accordingly, when the piston 7 descends during the scavenging stroke,the air-fuel mixture M within the crank chamber 2 a lubricates thebearings 81 as it enters the second scavenging passages 22 through thebearings 81. Also, during the intake stroke, the second reed valve 64provided in the air passage 11 shown in FIG. 24 is also opened, allowingthe air A from the second passage 62 in the adaptor 60 to be introducedinto the second cutout 1 e and then into the second scavenging passages22 through the associated second connecting pipes 66 shown in FIG. 28.

Therefore, only principally necessary amounts of the air-fuel mixture Mand the air A shown in FIG. 24 can be allowed to fill up the first andsecond scavenging passages 21 and 22, respectively. For this reason, itis possible to prevent the blow-off of an excessively enriched air-fuelmixture from entering the combustion chamber 1 a at the end of thescavenging stroke and then blowing off through the exhaust port 12 a.Also, during the scavenging stroke, the air A introduced into the secondscavenging passages 22 shown in FIG. 28 is first injected into thecombustion chamber 1 a, followed by the air-fuel mixture M beinginjected through the first scavenging passages 21 shown in FIG. 27.Since at this time the air-fuel mixture M behaves in such a manner thatthe enriched air-fuel mixture within the first scavenging passages 21enters the combustion chamber 1 a and, thereafter, the air-fuel mixtureM within the crank chamber 2 a, which is a leaned air-fuel mixture,enters the combustion chamber 1 a through the first scavenging passages21, the blow-off of the enriched air-fuel mixture can be avoided withthe charging efficiency increased consequently.

The two-cycle combustion engine according to a seventh preferredembodiment of the present invention will furthermore be described. Thistwo-cycle combustion engine is similar to that according to the thirdembodiment, but is featured in that as principal bearings for supportingthe crankshaft 8, needle bearings 51 are employed, and other structuralfeatures of the two-cycle combustion engine according to the seventhembodiment except a lower end position of the scavenging passages aresimilar to those according to the third embodiment.

FIG. 29 is a front sectional view showing the cylinder block and thecrankcase, and FIG. 30 is a side sectional view showing the cylinderblock and the crankcase. While in any one of the first to sixthembodiments a ball bearing has been employed for each of the bearings 81for supporting the crankshaft 8, in this seventh embodiment thecrankshaft 8 shown in FIG. 8 is rotatably supported by needle bearings51 and, in addition, thrust washers 52 are used to bear the thrust loadacting on the crankshaft 8. Considering that each of the needle bearings51 has, as a general property, an outer diameter smaller than that ofthe ball bearing, the first and second scavenging passages 23 and 24 areextended straight downwardly, as shown in FIG. 29, a distancecorresponding to the difference in outer diameter between the needlebearing 51 and the ball bearings.

In other words, while the second scavenging ports 23 a and 24 a at therespective upper ends of the first and second scavenging passages 23 and24, each employed in a pair, are arranged at a location substantiallylevel with those in the third embodiment (FIG. 18), inflow ports(openings) 23 b and 24 b at the respective lower ends thereof arelocated in the vicinity of radially outer sides of the adjacent needlebearings 51, that is, immediately there above, and formed in an arcuateshape following the curvature of an outer periphery of each of theneedle bearings 51. Also, the first and second scavenging passages 23and 24 are, as shown in FIG. 30, formed with small communicating holes23 c and 24 c for introducing an air-fuel mixture from respectivelocations adjacent the inflow ports 23 b and 24 b to the needle bearings51.

According to the foregoing construction in accordance with the seventhembodiment, when the negative pressure is developed inside the crankchamber 2 a shown in FIG. 29 during the intake stroke, the reed valve 32provided in the air passage 11 is opened to allow the air A flowingthrough the air passage 11 to be introduced from the air inflow ports 24b through the cutout 11 b, the air introducing ports 11 d, theconnecting pipes 30 (FIG. 30), the air discharge ports 11 e (FIG. 30)and the second scavenging passages 24 into regions adjacent radiallyouter sides of the needle bearings 51 within the crank chamber 2 a, thatis, into regions adjacent the crankshaft 8. At this time, as is the casewith the third embodiment (FIG. 18), during a period in which the reedvalve 32 is opened by the effect of the negative pressure inside thecrank chamber 2 a during the intake stroke, the air A is introduced intothe second scavenging passages 24 at all times and, since the secondscavenging passages 24 extend downwardly so large a distance that theyhave a large capacity, a sufficient amount of air for the prevention ofthe blow-off can be secured within the second scavenging passages 24. Onthe other hand, during the intake stroke, the air-fuel mixture M flowsthrough the air-fuel mixture passage 10 and is then introduced directlyinto the crank chamber 2 a, as shown by the arrows, from the air-fuelmixture ports 20 shown in FIG. 30, which open at the inner peripheralsurface of the cylinder block 1. The crankpin 82 is thus satisfactorilylubricated by the air-fuel mixture M so introduced.

During the subsequent scavenging stroke, the sufficient amount of theair accommodated within the second scavenging passages 24 isprogressively injected into the combustion chamber 1 a through thesecond scavenging ports 24 a and, thereafter, the leaned air-fuelmixture M, present in the vicinity of the radially outer portions of theneedle bearings 51, that is, in a center portion of the crank chamber 2a, flows from the inflow ports 23 a into the first scavenging passages23 and is then injected into the combustion chamber 1 a through thefirst scavenging ports 23 a, and at the end of the scavenging stroke,the enriched air-fuel mixture M, urged to a region adjacent an innerwall of the crank chamber 2 a by the effect of a centrifugal forcedeveloped by the rotation of the crank webs 84, is introduced into thecombustion chamber 1 a in a delayed fashion. Because of these, theblow-off of the air-fuel mixture M can be further effectivelysuppressed. At this time, a portion of the air-fuel mixture M within thecrank chamber 2 a enters the needle bearings 51 from the inflow ports 23b and 24 b and then through the first and second scavenging passages 23and 24 and the associated communicating holes 23 c and 24 c, therebylubricating the needle bearings 51.

In this embodiment, since the first and second scavenging passages 23and 24 can be formed as a straight passage while being extended adistance downwardly, as compared with the case in which passages arecurved and are then extended downwardly so as to detour large ballbearings, the flow resistance in the passage and the loss of output canbe reduced, the manufacture is easy to carry out and, since the needlebearings 51 are lighter than the ball bearings, the engine body can bemanufactured lightweight.

Hereinafter, the two-cycle combustion engine according to an eighthpreferred embodiment of the present invention shown in FIGS. 31 and 32will be described. This two-cycle combustion engine is similar to thataccording to the seventh embodiment shown in FIG. 30, but is featured inthat the crank webs 84 are used as a valve so that the timings of thescavenging with the air and the air-fuel mixture can be controlled bythe crank webs 84, and other structural features of the two-cyclecombustion engine according to the eighth embodiment are similar tothose according to the seventh embodiment.

The two-cycle combustion engine according to the eighth embodimentdiffers from that according to the seventh embodiment in that, as shownin FIG. 32, the respective lower ends of the first and second scavengingpassages 23 and 24 are extended centrally of the crank case 2 ascompared with those in the seventh embodiment (FIG. 30) with the inflowports 23 b and 24 b positioned as close to respective outer side faces84 a of the crank webs 84 as possible and in that those inflow ports 23b and 24 b are so formed as to be of an arcuate shape following thecurvature of the outer periphery of the respective needle bearing 51 asshown in FIG. 31 and longer than those in the seventh embodiment (FIG.29). The inflow port 24 a for the air A has a shape longer than that ofthe inflow port 23 a for the air-fuel mixture M. Accordingly, in thistwo-cycle combustion engine, the crank webs 84 function as respectivevalves capable of selectively opening and closing the inflow ports 23 band 24 b as they rotate, and the inflow ports 23 b and 24 b are formedto represent an arcuate shape capable of being selectively opened andclosed at a predetermined timing according to the rotation of the crankwebs 84. Also, the respective upper edges of the second scavenging ports23 a and 24 a of the first and second scavenging passages 23 and 24 arepositioned at the same height.

The operation of the two-cycle combustion engine according to the eighthembodiment will now be described with reference to the timing chartshown in FIG. 33. When during the intake stroke the piston 7 shown inFIG. 31 arrives at the top dead center (TDC) with the cranking anglebeing 360° (0°), the reed valve 32 is opened as shown in FIGS. 33( a)and (c), and the inflow ports 24 b of the second scavenging passages 24are partially opened by the crank webs 84. Accordingly, as the negativepressure is developed inside the cylinder block 1 and the crank chamber2 a, the air A flowing in the air passage 11 can be introduced from theair introducing port 11 d into a region radially outwardly of the needlebearings 51, that is, from the inflow ports 24 b in the vicinity of thecrankshaft 8 into the crank chamber 2 a, through the connecting pipes30, the air discharge ports 11 e and the second scavenging passages 24.At this time, as is the case with the third embodiment (FIG. 18), duringthe period in which the reed valve 32 (FIG. 31) is opened in response tothe negative pressure inside the crank chamber 2 a during the intakestroke, the air A is kept introduced into the second scavenging passages24 at all times and, on the other hand, since the second scavengingpassages 24 extend so large a distance to have a large capacity, asufficient amount of air for the prevention of the blow-off can besecured within the second scavenging passages 24.

On the other hand, during the intake stroke, as shown in FIGS. 33( a)and (c), since the air-fuel mixture port 20 in FIG. 32 is opened and theinflow ports 23 b of the first scavenging passages 23 are opened by thecrank webs 84, in response to the negative pressure developed inside thecrank chamber 2 a the air-fuel mixture M flowing in the air-fuel mixture10 shown in FIG. 31 is introduced directly into the crank chamber 2 a,as shown by the arrows in FIG. 32, through the air-fuel mixture port 20opening in the inner peripheral surface of the cylinder block 1. Thecrankpin 82 is satisfactorily lubricated by the air-fuel mixture M sointroduced.

During the subsequent scavenging stroke, as shown in FIG. 33( e), theexhaust port 12 a shown in FIG. 31 starts opening at the timing at whichthe cranking angle attains substantially 100° and, although at thistime, as shown in FIG. 33( c), the inflow ports 24 a for the air A ofthe second scavenging passages 24 are opened, the inflow ports 23 b forthe air-fuel mixture M of the first scavenging passages 23 are closed bythe crank webs 84 as shown in FIG. 33( b). Further, as shown in FIG. 33(d), the first and second scavenging ports 23 a and 24 a are both closeduntil the cranking angle attains substantially about 130°. Accordingly,during a period in which the cranking angle is 100 to 130°, the air Awithin the second scavenging passages 24 are compressed by the effect ofthe pressure exerted by the piston 7 then descending and, at the timethe second scavenging ports 24 a are opened, only the air A compressedwithin the second scavenging passages 24 can be injected at high speedinto the combustion chamber 1 a, with the combustion chamber 1 a quicklyscavenged consequently. Since the sufficient amount of the air A isaccumulated within the second scavenging passages 24, the air-fuelmixture M is not entangled in the leading flow of the air A toeffectively suppress the blow-off.

Thereafter, when the piston 7 descends to the bottom dead center (BDC),the inflow ports 23 b for the air-fuel mixture M are opened as shown inFIG. 33( b) and, at the time the piston 7 has past the bottom deadcenter (BDC), the inflow ports 24 b for the air A are closed as shown inFIG. 33( c). Accordingly, within the combustion chamber 1 a having beensubstantially completely scavenged, the air-fuel mixture M within thecrank chamber 2 a is injected at high speed from the first scavengingports 23 a into the combustion chamber 1 a through the inflow ports 23 bby way of the first scavenging passages 23, with the charging efficiencyof the air-fuel mixture M into the combustion chamber 1 a increasedconsequently.

In this embodiment, since as hereinabove described the inflow ports 23 bfor the air-fuel mixture M, and the inflow ports 24 b for the air A bothopening in the crank chamber 2 a, are controlled by the crank webs 84with the latter inflow ports 24 b opened prior to the former inflowports 23 b, the respective upper edges of the first and secondscavenging ports 23 a and 24 a are set at the same heights to therebyallow them to be opened on the same timing during the descending motionof the piston 7. This scavenging system is more effective than thepiston valve system in which the respective positions of the upper endsof the scavenging ports for the air-fuel mixture and the air,respectively, are offset relative to each other. In other words, withthe piston valve system, the pressure inside the crankcase at the timethe scavenging ports for the air are opened are lower than the pressureinside the crankcase at the time the scavenging ports for the air-fuelmixture and, therefore, it tends to occur that neither a quickscavenging of the combustion chamber with the air nor the suppression ofthe blow-off can be accomplished effectively.

Now the two-cycle combustion engine according to a ninth preferredembodiment of the present invention will be described with reference toFIGS. 34 and 35. This two-cycle combustion engine is featured in thatthe crankcase 2 shown in FIG. 34 is of two-piece construction to allowthe second scavenging passages 24 to be extended downwardly a furtherdistance than those in the seventh embodiment (FIG. 29), and otherstructural features of the two-cycle combustion engine according to theninth embodiment are similar to those according to the seventhembodiment.

As clearly shown in FIG. 34, the crankcase 2 is of the two-piececonstruction including an upper casing body 2A and a lower casing body2B connected together, and the second scavenging passages 24 areconstructed of respective passage portions that are defined in the upperand lower casing bodies 2A and 2B and communicated with each other. Thesecond scavenging passages 24 have lower end portion formed to extenddownwardly around the needle bearings 51 so that the inflow ports 24 bof the second scavenging passages 24 are opened at respective positionsradially below the needle bearings 51. The inflow ports 23 b at thelower ends of the first scavenging passages 23 are opened at respectivepositions higher than those in the seventh embodiment (FIG. 29). Otherstructural features of the two-cycle combustion engine according to theninth embodiment are similar to those according to the seventhembodiment.

According to the foregoing construction in accordance with the ninthembodiment, since the second scavenging passages shown 24 in FIG. 35extend down to the position radially of the needle bearings 51, evenwhen the number of revolutions of the engine is increased, a sufficientamount of air required to avoid the blow-off can be secured within thesecond scavenging passages 24 during the intake stroke. On the otherhand, the air-fuel mixture M can be, during the intake stroke,introduced directly into the crank chamber 2 a, as shown by the arrows,through the air-fuel mixture port 20 open in the inner peripheralsurface of the cylinder block 1. The crankpin 82 can be satisfactorilylubricated by the air-fuel mixture so introduced.

During the subsequent scavenging stroke, at the time when the inflowport 24 b of the second scavenging passage 24 is opened by the crankwebs 84, a portion of the air-fuel mixture M in the crank chamber 2 aenters into the needle bearings 51 through the communication holes 24 cto thereby lubricate the needle bearings 51.

It is to be noted that although in any one of the seventh to ninthembodiments, the example has been shown which utilizes such basicstructure as in the third embodiment, the essential structure of theseembodiments, in which the crankshaft 8 is supported by the needlebearings 51 and at least the second scavenging passages 23 and 24 areextended further downwards can be equally applied to the two-cyclecombustion engine according to any one of the first, second and fourthto sixth embodiments, but the structure in which the first or secondscavenging passages are communicated with the crank chamber through thebearings is eliminated therefrom. Where the foregoing essentialstructure is applied to the first embodiment, the air-fuel mixture is,during the intake stroke, introduced into the first scavenging passages,not into the crank chamber, and the air is introduced into the crankchamber. In addition, although not encompassed within the presentinvention, the foregoing important structure can be applied to anystandard two-cycle combustion engine of a type in which the scavengingis performed with only the air-fuel mixture introduced into thecombustion chamber, other than the type in which the scavenging with theair A is initially performed prior to the scavenging with the air-fuelmixture M. Where the foregoing important structure is applied to thisstandard two-cycle combustion engine, it is possible to introduce thelean air-fuel mixture, urged to a region adjacent an inner wall of thecrank chamber, into the combustion chamber after the lean air-fuelmixture within a center portion of the crank chamber has first beeninjected into the combustion chamber prior and, therefore, the blow-offof the air-fuel mixture can be suppressed.

1. A two-cycle combustion engine which comprises: a combustion chamber;a crank chamber accommodating a crankshaft; a bearing provided on a sidewall of the crank chamber for rotatably supporting the crankshaft; afirst scavenging passage for supplying an air-fuel mixture to thecombustion chamber; and a second scavenging passage for supplying ascavenge air to the combustion chamber; wherein one of the first andsecond scavenging passages communicates between the combustion chamberand the crank chamber through the bearing for the crankshaft, and theother of the first and second scavenging passages communicates directlybetween the combustion chamber and the crank chamber.
 2. The two-cyclecombustion engine as claimed in claim 1, wherein the first scavengingpassage communicates between the combustion chamber and the crankchamber through the bearing for the crankshaft while the secondscavenging passage communicates directly between the combustion chamberand the crank chamber, and further comprising: a suction chamber formedin a side face of a piston; an air-fuel mixture passage for introducingan air-fuel mixture M into the suction chamber; and an air passage forintroducing an air into the crank chamber; wherein during an intakestroke of the engine, the air-fuel mixture from the air-fuel mixturepassage is introduced into the first scavenging passage through thesuction chamber and the air from the air passage is introduced into thecrank chamber; and wherein during a scavenging stroke of the engine,introduction of the air within the crank chamber into the combustionchamber through the second scavenging passage takes place before theair-fuel mixture within the first scavenging passage is introduced intothe combustion chamber.
 3. The two-cycle combustion engine as claimed inclaim 2, wherein the piston has a lubricant passage formed therein forsupplying the air-fuel mixture within the suction chamber to a small endbearing disposed between a piston pin and a connecting rod.
 4. Thetwo-cycle combustion engine as claimed in claim 2, wherein the secondscavenging passage is positioned at a location closer to an exhaust portopening to the combustion chamber for discharging an exhaust gas fromthe combustion chamber than the first scavenging passage in a directioncircumferentially of the combustion chamber.
 5. The two-cycle combustionengine as claimed in claim 4, Further comprising a third scavengingpassage for communicating directly between the combustion chamber andthe crank chamber; the third scavenging passage being positioned at alocation closer to the exhaust port than the second scavenging passagein the direction circumferentially of the combustion chamber; andwherein during the scavenging stroke, introduction of the air within thecrank chamber into the combustion chamber through the second scavengingpassage takes place before an air-fuel mixture introducing timing, atwhich the air-fuel mixture within the first scavenging passage isintroduced into the combustion chamber, and, simultaneously with theair-fuel mixture introducing timing or at a timing thereafter,introduction of the air within the crank chamber through the thirdscavenging passage takes place.
 6. The two-cycle combustion engine asclaimed in claim 5, wherein an opening of the second scavenging passagetowards the crank chamber is closed by the piston before the pistonreaches a bottom dead center.
 7. The two-cycle combustion engine asclaimed in claim 1, wherein the first scavenging passage communicatesdirectly between the combustion chamber and the crank chamber while thesecond scavenging passage communicates between the combustion chamberand the crank chamber through the bearing for the crankshaft, andfurther comprising: a suction chamber formed in a side face of a piston;an air passage for introducing an air into the suction chamber; and anair-fuel mixture passage for introducing an air-fuel mixture into thecrank chamber; wherein during an intake stroke of the engine, the airfrom the air passage is introduced into the second scavenging passagethrough the suction chamber and the air-fuel mixture from the air-fuelmixture passage is introduced into the crank chamber; and wherein duringa scavenging stroke of the engine, introduction of the air within thesecond scavenging passage into the combustion chamber takes place beforethe air-fuel mixture within the crank chamber is introduced into thecombustion chamber through the first scavenging passage.
 8. Thetwo-cycle combustion engine as claimed in claim 7, further comprising anair regulating valve for closing the air passage when a pressure insidethe air passage decreases to a value equal to or lower than apredetermined value.
 9. The two-cycle combustion engine as claimed inclaim 7, wherein an opening of the first scavenging passage towards thecrank chamber is closed by the piston before the piston reaches a bottomdead center.
 10. The two-cycle as claimed in claim 7, wherein the secondscavenging passage is positioned at a location closer to an exhaust portopening to the combustion chamber for discharging an exhaust gas fromthe combustion chamber than the first scavenging passage in a directioncircumferentially of the combustion chamber.
 11. The two-cyclecombustion engine as claimed in claim 1, wherein the first scavengingpassage communicates directly between the combustion chamber and thecrank chamber while the second scavenging passage communicates betweenthe combustion chamber and the crank chamber through the bearing for thecrankshaft, and further comprising: an air passage for introducing anair into the second scavenging passage; a reed valve disposed in the airpassage; and an air-fuel mixture passage for introducing an air-fuelmixture into the crank chamber; wherein during an intake stroke of theengine, the air from the air passage is introduced into the secondscavenging passage through the reed valve and the air-fuel mixture fromthe air-fuel mixture passage is introduced into the crank chamber; andwherein during a scavenging stroke of the engine, introduction of theair within the second scavenging passage into the combustion chambertakes place before the air-fuel mixture within the crank chamber isintroduced into the combustion chamber through the first scavengingpassage.
 12. The two-cycle engine as claimed in claim 11, wherein anopening of the first scavenging passage towards the crank chamber isclosed by the piston before the piston reaches a bottom dead center. 13.The two-cycle engine as claimed in claim 11, wherein the secondscavenging passage is positioned at a location closer to an exhaust portopening to the combustion chamber for discharging an exhaust gas fromthe combustion chamber than the first scavenging passage in a directioncircumferentially of the combustion chamber.
 14. The two-cyclecombustion engine as claimed in claim 1, wherein the first scavengingpassage communicates directly between the combustion chamber and thecrank chamber while the second scavenging passage communicates betweenthe combustion chamber and the crank chamber through the bearing for thecrankshaft, and further comprising: an air-fuel mixture passage forintroducing an air-fuel mixture into the first scavenging passage; anair passage for introducing an air into the second scavenging passage; afirst reed valve disposed in the air-fuel mixture passage; a second reedvalve disposed in the air passage; wherein during an intake stroke ofthe engine, the air-fuel mixture from the air-fuel mixture passage isintroduced into the first scavenging passage and the air from the airpassage is introduced into the second scavenging passage; and whereinduring a scavenging stroke of the engine, introduction of the air withinthe second scavenging passage into the combustion chamber takes placebefore the air-fuel mixture within the first scavenging passage isintroduced into the combustion chamber.
 15. The two-cycle engine asclaimed in claim 14, wherein the second scavenging passage is positionedat a location closer to an exhaust port opening to the combustionchamber for discharging an exhaust gas from the combustion chamber thanthe first scavenging passage in a direction circumferentially of thecombustion chamber.
 16. A two-cycle combustion engine, which comprises:a needle bearing for supporting a crankshaft within a crankcase; firstand second scavenging passages for communicating between a combustionchamber and a crank chamber; an air-fuel mixture passage for introducingan air-fuel mixture into the crank chamber or the first scavengingpassage during an intake stroke; an air passage for introducing an airinto the second scavenging passage or the crank chamber during theintake stroke; and a communicating hole for fluidly connecting the firstor second scavenging passage with the needle bearing; wherein during ascavenging stroke of the engine, introduction of the air within thesecond scavenging passage into the combustion chamber takes place priorto the air-fuel mixture within the first scavenging passage beingintroduced into the combustion chamber; and wherein an opening of alower end of the second scavenging passage towards the crank chamber ispositioned at a location adjacent a region radially outwardly of theneedle bearing.
 17. The two-cycle combustion engine as claimed in claim16, wherein an opening of a lower end of the first scavenging passagetowards the crank chamber is positioned at a location adjacent a regionradially outwardly of the needle bearing.